There are many potential military and commercial applications for a passive magnetic sensing system that can detect, track and measure the DC magnetic anomaly fields (or signatures) of moving magnetic objects or “targets” in real-time. The word “passive” indicates that the magnetic sensing system does not act to produce magnetic signatures but only detects (and analyzes) a target's inherent magnetic signature. The magnetic signatures are produced in and emanate from the ferrous materials (e.g., steel) that are contained in the physical structure of a target. Targets of interest that produce detectable magnetic signatures include watercraft such as naval vessels and land vehicles such as cars, trucks or military tanks. Frequently, the presence, location, state of motion, and magnetic signature or identity (i.e., classification) of these targets must be determined.
Accurate detection, ranging and classification of magnetic objects usually requires a number of vector magnetic sensors that are configured as magnetic gradiometers. A gradiometer measures magnetic gradients, i.e., the rates of change of magnetic fields with distance. It is known in the art that passive magnetic detection and ranging of moving targets can be achieved by using a stationary magnetic sensing system having a combination gradiometer/magnetometer that measures five independent gradient tensor components and a set of vector field components of the target's magnetic anomaly field. However, prior art approaches have not produced a practical system that can be readily and effectively deployed. The limitations of conventional prior art magnetic detection and ranging systems include the following:
(i) Use of target localization methods that are rather complex, computationally intensive and difficult to implement in a cost effective and easily deployable system.
(ii) The effective response to a fast-moving target may be too slow because the prior art approaches typically may require a significant “time-series” of data be taken before a target can unambiguously be located. Thus, the sensor system response may lag behind the actual position of the target.
(iii) The accuracy may be reduced if a target's magnetic signature changes with the target's motion. That is, prior art approaches for target ranging and classification usually assume that target's magnetic signature vector remains constant while the target is being tracked.
Recently, U.S. Pat. No. 6,476,610 (i.e., “the '610 patent” as it will be referred to hereinafter) disclosed a novel magnetic gradiometer and signal processing concept denoted as “Scalar Triangulation and Ranging” (STAR) for target localization from maneuverable sensing platforms. The STAR method uses unique, rotationally invariant scalar “contractions” of magnetic gradient tensor components to “triangulate” relative distances to a target. Within the target-detection distance of a STAR-type gradiometer, the scalar triangulation process does not directly depend on the target's magnetic dipole strength.
More recently, U.S. Pat. No. 6,841,994 (i.e., “the '994 patent” as it will be referred to hereinafter) disclosed significant improvements to the STAR design and method that better determine the range, relative bearing and magnetic signature of a stationary target from a mobile sensing platform. However, the '610 patent and the '994 patent primarily address the problem of detection and ranging of fixed targets with constant magnetic signature vectors. Thus, in terms of tracking a moving target, the teachings of the '610 and '994 patents have the following limitations:
(i) Their general target-ranging methods may yield inaccurate or ambiguous results for certain sensor orientations relative to the target.
(ii) They do not disclose effective operational modalities for the general case of a moving target whose velocity and magnetic dipole signature may be changing with time.
(iii) They do not take into account “asphericity errors” of the gradient contraction-based STAR method that may reduce the accuracy of target ranging and signature measurements made using the STAR method.
The above-mentioned “asphericity errors” will now be explained with the aid of FIGS. 1A and 1B. As disclosed in the '610 and '994 patents, the gradient contraction CT2 of the full, nine-component magnetic gradient tensor is a rotationally invariant and robust scalar that is independent of gradiometer orientation. The mathematical and geometrical properties of CT are graphically represented in FIG. 1A where contours of constant CT (qualitatively represented by dashed contour lines 300) form concentric prolate spheroidal surfaces that enclose a source (e.g., target T) of the magnetic anomaly field. The dipole axis of the field is illustrated by dotted line 302 and the field's transverse or “equator” axis is illustrated by dotted line 304. Magnetic lines of force of the dipole field are illustrated by solid force lines 400.
Additional reference will now be made to FIG. 1B where only one of contours 300 is illustrated for clarity of description. At a given sensor-target distance “r”, CT is primarily a function of the magnetic dipole moment M of target T, distance r, and a dimensionless asphericity parameter “k”. The k-parameter characterizes the variance of CT (owing to the prolate spheroidal nature of contours 300) from true spherical symmetry. Specifically, for media with constant magnetic permeability μ, CT=k(μ/4π)M/r4 where calculations show that k slowly varies from approximately 7.3 for points aligned with the dipole axis 302 to 4.2 for points on equator 304. Conversely, for contours of constant gradient contraction, the ratio of the diameter at dipole axis 302 to the diameter at equator 304 is only approximately 1.14 to 1.
The aspherical nature of the constant CT contours can cause inaccuracies or “asphericity errors” in the STAR methods described in the '610 and '994 patents. For example, referring to FIG. 1B, for a sensor position P located between dipole axis 302 and equator 304, application of the STAR method produces a measured vector position or range r′ and a measured target dipole moment M′ that can differ slightly from the true values of range r to target T and the dipole moment M thereof.